Although numerous studies have tested the effects of continuous exercise regimens on antioxidant defences, information on the effect of sprint exercise on the antioxidant defence system and lipid peroxidation levels of tissues is scant. The present study was designed to determine the effects of sprint exercise on the lipid peroxidation and antioxidant enzyme system in liver and skeletal muscle during the post-exercise recovery period in untrained mice. Mice performed 15 bouts of exercise, each comprising running on a treadmill for 30s at 35 m.min(-1) and a 5degrees slope, with a 10-s rest interval between bouts. They were then killed by cervical dislocation either immediately (0 h), 0.5 h, 3 h or 24 h after completion of the exercise. Their gastrocnemius muscle and liver tissues were quickly removed. It was found that blood lactate levels increased immediately after the exercise, but had returned to control levels by 0.5 h postexercise. This exercise regimen had no effect on the activity of superoxide dismutase and glutathione peroxidase in these tissues. Levels of muscle thiobarbituric acid reactive substances (TBARS) had increased at 0.5 and 3 h post-exercise, and then returned to control levels by 24 h post-exercise. In conclusion, acute sprint exercise in mice resulted in an increase in TBARS levels in skeletal muscle; no change was observed in the liver. Antioxidant enzyme activities remained unaffected by acute sprint exercise in these tissues.